Local area networks (LANs) to enable local communication among computing stations and computer related devices are well known in the art. The reference entitled "An Introduction to Local Area Networks" by D. Clark et al, Proceedings of the IEEE, Vol. 66, No. 11, pp. 1497-1517, November 1978, provides a general description of local area networks and their operation and is incorporated in its entirety herein by reference. Generally, message packet switching schemes are employed on LAN's to enable transmission of information between devices connected to the LAN. Data is packetized and formed into message frames which include, in addition to the information being transmitted, device source and destination addresses that respectively identify the device transmitting the message and the device to which the message is being transmitted. The devices connected to a LAN normally have unique address identifications to enable such source and destination addressing. The manner in which user information is transmitted between devices on the LAN and the method for properly formating the message frames are well known in the art and do not form part of the present invention.
LANs are implemented using a variety of network topologies and control structures, the control structure determining how the different devices connected to a network may achieve access thereto. Such topologies and control structures are described in the above-cited reference by Clark et al.
For a variety of reasons, described in Clark et al., it is desirable under certain circumstances to interconnect discrete LANs by means of communication bridges. A bridge interconnecting two LANs acts as a filter by transmitting only message frames, received on one side of the bridge, having a destination address of a device located on the other side of the bridge. A bridge includes, typically, two transceivers, each transceiver operating to transmit message frames to and receive frames from one of the two LANs the bridge interconnects. Note that while the transceivers are essential components that enable receipt and transmission of message frames, they are not always considered an integral part of a bridge by those skilled in the art. Bridges also include means for interfacing with the respective control structures of the LANs they interconnect. Additionally, bridges include means for performing the message frame filtering discussed above.
In order for a bridge to effectively filter message frames, it must know the address identities of the devices located on the LANs it interconnects. In general, in the case of a communication system comprising multiple LANs interconnected by bridges, each bridge must know the address identities of all devices in the system which communicate between LANs and the respective locations of such devices relative to the bridge. FIG. 1 illustrates a communication system 50 comprising LANs 52, 54 and 56 as well as other LANs that system 50 may include but that are not shown. Bridges 58 and 60 respectively interconnect LANs 52 and 54 and LANs 54 and 56. Note that LANs are generally not interconnected by bridges so as to form a loop. For illustrative purposes, it is helpful to view bridge 58 as consisting of sides 62 and 64 and bridge 60 as consisting of sides 66 and 68. Then, bridge 58 needs to know the locations of devices on system 50 relative to its bridge sides 62 and 64 while bridge 60 needs to know device locations relative to its bridge sides 66 and 68. For example, bridge 58 needs to know that devices connected to LANs 54 and 56, as well as devices connected to LANs beyond LAN 56, are located on its side 64 while devices on LAN 52 and LANs beyond LAN 52 are located on its side 62. Thus, if a message frame transmitted by a source device on LAN 52 has as its destination a device connected to LAN 56, bridge 58, in determining whether to transmit the message frame through, needs to know that the source device is on bridge side 62 and the destination device is located on side 64.
Typically, a bridge includes means for storing a list of device address identifications, and their respective locations relative to the sides of the bridge, in order to enable the bridge to filter message frames. This poses an immediate problem should devices be added to or deleted from a LAN or if a device is moved from one LAN to another relative to the sides of any bridge in the communication system. Such changes need to be accounted for in order to maintain the operating efficiency of the bridge and the LANs it interconnects. One solution to the problem is to have a human system administrator keep track of such changes and effect appropriate adjustments in the respective bridge list storing means. Such a solution requires record keeping and periodic assessment of device identities on the interconnected LANs.
Another solution, known in the art, is to provide a bridge which upon initial operation of the communication system enters a listening period during which it accumulates but does not transmit message frames. The bridge reads the source and destination addresses of the accumulated message frames and forms a database of device addresses and locations relative to the sides of the bridge. When the bridge leaves the listening mode, it determines, in accordance with the device address database, which of the accumulated frames require transmission and so dispositions them. The bridge determines in the same fashion whether or not to transmit subsequently arriving message frames. With respect to the addition of new devices to the LANs in the communication system, it is necessary for the new device to broadcast an introductory "Hello" message to apprise the bridges and devices comprising the system of its presence. The bridge is adapted to be responsive to the "Hello" message to update its address database.
One drawback to the above solution is the substantial transmission delay, during initial operation, resulting from the listening period. A second drawback is the need to generate a "Hello" message in order to update the bridge address database. With reference to the International Standards Organization (ISO) seven layer open systems interconnection model, the "Hello" message would have to be generated by the new device within a protocol layer above the datalink layer. This is undesirable since it is generally preferred to maintain all aspects of bridge operation at and below the datalink layer and thus independent of the higher protocol layers.